1. Field of Invention
This invention relates generally to the measurement of minute currents in the sub-nanoampere range flowing through extremely high impedances, and more particularly to a digitallynulled high-impedance current measuring bridge system which functions to null out parasitic and other spurious currents to provide precise current measurements.
2. Prior Art
The concern of the present invention is with the measurement of minute bias currents which flow in semiconductor devices having an extremely high impedance. While the invention is useful in measuring currents in unipolar transistors and other high impedance devices, for purposes of explanation it will be described in conjunction with a device under test (DUT) which takes the form of a linear operational amplifier having a fieldeffect transistor (Bi-FET) input stage. A measuring system in accordance with the invention takes into account certain problems which are encountered in such amplifiers and is therefore particularly useful in the testing thereof.
A solid-state operational amplifier is a direct-coupled high-gain amplifier that is designed to use external feedback to control its feedthrough characteristics. Because an amplifier of this type is generally used to perform a wide variety of linear functions, it is often referred to as the basic linear integrated circuit. The integrated operational amplifier has gained broad acceptance as a versatile and predictable system building block, for it affords all of the advantages of monolithic integrated circuits, including reduced cost, temperature tracking and low offset voltage and current.
Operational amplifiers have a differential input, a voltage V.sub.2 being applied to the inverting or (-) input and a voltage V.sub.1 being applied to the non-inverting or (+) input. In an ideal operational amplifier, the input impedance has an infinite value and a zero output voltage V.sub.o when input voltage V.sub.1 equals input voltage V.sub.2. But a real operational amplifier exhibits offset error-voltage and currents as a result of a mismatch of the input transistors. This mismatch gives rise to unequal bias currents flowing through the input terminal. It requires, therefore, the application between the two input terminals of an input offset voltage to balance the amplifier output.
In an operational amplifier, the input bias current I.sub.B is part of the sum of the separate currents entering the two input terminals of a balanced amplifier. Thus the input bias current I.sub.B equal I.sub.B1 plus I.sub.B2 divided by 2, when output voltage V.sub.o =0. The input offset current I.sub.IO is the difference between the separate currents entering the input terminals of a balanced amplifier. Hence I.sub.IO =I.sub.B1 -I.sub.B2, when V.sub.o =0. The input offset current drift is the ratio of the change of input offset current to a change in temperature.
The metal-oxide-silicon, insulated-gate, field-effect transistor (MOS-FET) is quite different both in its structure and characteristics from the conventional bipolar junction transistor, and its unique properties are especially useful in operational amplifiers. One important characteristic of the field-effect transistor is its extremely high input impedance at low frequencies, which makes it the semiconductor counterpart of a vacuum-tube triode. Essentially, there is no d-c coupling through the insulated gate, and typical impedances are in the order of 10.sup.15 ohms.
For the purpose of testing operational amplifiers for input bias current, several techniques are presently used. These known techniques, some of which will later be described, are incapable of precisely measuring the bias current in a Bi-FET operational amplifier. The bias currents which flow through such extremely high input impedances lie in the sub-nanoampere or picoampere range, and the measurements carried out by existing techniques are uncertain in that they fail to take into account spurious components introduced by leakage and parasitic currents. Also ionization conditions prevailing in the atmosphere in which the device is being tested may give rise to an ionization current flow at the terminals of the amplifier.
The spurious current component is extremely small; but since the bias current through the high-impedance device under test lies in the sub-nanoampere range, the spurious current component represents a significant fraction of the total current flow. Unless the spurious current component is taken into account, it gives rise to a misleading result.